Metal vapor lamp with alkali metal reservoir means

ABSTRACT

This invention contemplates the use of a porous beryllium plug, having an oxidized surface, which is located in and adjacent to, or on the cooler end of an alkali metal vapor lamp to serve as a reservoir for the alkali metal.

nited States Patent Inventor Appl. No,

Filed Patented Assignee The United States of America as represented by the Secretary of the Army METAL VAPOR LAMP WITH ALKALI METAL UNITED STATES PATENTS Charlton Druyvesteyn et a1. Slack et a1.

References Cited Ekkers et al.

Fridn'ch Louden Knochel et al Primary Examiner-Roy Lake Assistant ExaminerPalmer C. Demeo 313/178 3l3/227X 313/178 3l3/227X 313/217 3l3/227X 3l3/184X Attorneys-Harry M. S'aragovitz, Edward J. Kelly, Herbert Berl and Milton M. Davis US. Cl. 313/174, 313/178, 313/220, 313/227 Int. Cl ..1-l01j 61/22, ABSTRACT: This invention contemplates the use of a porous H01 j 61/28 beryllium plug, having an oxidized surface, which is located in Field of Search 3 1 3/ 178, and adjacent to, or on the cooler end of an alkali metal vapor 179, 174, 220, 227, 184 lamp to serve as a reservoir for the alkali metal.

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I 1 l I 1! //1 'A\/, I I III/ l/Il a I ,I,', If --t i- W L 23 2| 25 J PATENTEDSEP14-l97l 3.16 o 4" 97 2 INVENTOR. STEPHEN LEVY MQM, wim y, flw

ATTORNEYS.

METAL VAPOR LAMP WITH ALKALI METAL RESERVOIR MEANS The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

BACKGROUND OF THE INVENTION The present invention relates generally to alkali metal vapor lamps and more specifically to a means for prolonging the operating life of the lamp as hereinafter described.

The principal limiting factor in lamps using the vapors of the alkali metals of group 1A, that is sodium, potassium, rubidium and cesium, has generally been the inability of the envelope material to withstand the attack of the metal vapors at the temperatures required for the efficient operation of the lamp. These metal vapors attack varied kinds of envelope materials and cause rapid blackening, or often cracking, at the temperatures at which the envelopes must be operated to achieve substantial light output. In the past several years, the use of cesium has become widespread and most favorable for the operation of metal vapor lamps. However, it has been found that cesium attacks envelopes even at temperatures in the neighborhood of 500 C. to blacken or cause rupture of the envelope.

In the past several years the use of alumina, or aluminum oxide has become extensive for the envelope or tube material in alkali metal lamps. However, it has been found that envelopes made of such material have poor severe thermal conductivity making such lamps susceptible to thermal shock which can destroy the tubular envelope or jacket when there is an abrupt change in temperature. Such severe thermal shock can be brought about by condensing cesium on the envelope during startup or shutdown periods.

In addition to the operating problems set forth above, it has been discovered that other factors have arisen that tend to shorten lamp life. One such factor is that the cold spot should be retained as the bottom electrode. If lamps, as presently designed, are mounted in an airplane and subjected to invert operation, the liquid cesium drops toward the lower end. Accordingly, this can cause lamp instability to effect damage to the lamp. It has been found that on shutdown the alkali metal may collect on the envelope to cause startup problems.

SUMMARY OF THE INVENTION It is the general purpose of the present invention to provide a means for preventing alkali metal buildup or concentration on the ceramic envelope. To attain this, the present invention contemplates the use of a porous beryllium plug, having an oxidized surface and having a surface area sufficient to hold all, or most of the alkali metal. The plug, or core, is located inside the cooler end of the lamp and in good thermal contact therewith.

BRIEF DESCRIPTION OF THE DRAWING The exact nature of this invention will be readily apparent from consideration of the following specification relating to the annexed drawing in which:

The single FIGURE is a sectional view of a cesium lamp embodying the invention herein.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing there is shown in the drawing an alkali metal lamp that includes an envelope or jacket 11 and end caps or closures 13, I5. Extending through the cap 13 is a tube 17, brazed to the cap to make a hermetic seal. The tube 17 serves to support a conventional electrode 21 filled with activating material in the form of alkaline earth oxides.

Another electrode 23 is supported in substantially the same manner at the other end of the envelope as shown. No perforation of the end cap I5 is required for support of the electrode 23 as the electrode is secured indirectly to the inner surface of the end cap I5.

The tube 17 is used to evacuate the lamp and to introduce the ionizable medium therein including an inert starting gas such as Argon and a quantity of Cesium. The method of introducing the Cesium, and the structure of the lamp described above are well known in the art, and it is believed that no further description thereof is necessary.

The novel improvement of the present invention resides in the addition of a Beryllium annular plug 25, which in the adaptation shown in the drawing is seen to be spaced from the pinched end of tube 17 but generally encompassing a portion of the tube 17 confined in the lamp. The plug 25 is preferably of porous construction which has had its surface oxidized to form an oxide layer, say of Beryllium oxide, of about 1,000 angstroms. Such porous oxidized surface presents a relatively extremely large surface area sufficient to hold and retain all the alkali metal, such as Cesium. This is due to the fact that the alkali metal will adhere more readily and rapidly to the beryllium oxide film, at a lower temperature, than to a metal surface, such as the interior surface of the lamp end caps l2, 15. The design and size of the porous plug 25 can be readily determined so that it is sufficient to hold all the alkali metal. In addition to the physical characteristics heretofore attributed to the plug 25, it is also desirable that the plug be made of a material that is compatible with all the components in the lamp, have high thermal conductivity and have a high melting point, that is greater than 700 C. In addition, the oxide film on the plug should be a good insulator at 650 C. In the specific construction described it has been indicated that the oxide layer is of beryllium oxide. It is to be understood however that other ceramic oxides, such as aluminum oxide or the like can be utilized. While in the specific embodiments shown herein indicate that the reservoir be in the form of a plug 25, it is obvious that other forms of construction can be utilized. Thus, it is contemplated that the reservoir can be in the nature of a wrapped wire or wire gauze, both suitably oxidized as heretofore indicated.

In the operation of an alkali metal lamp with an inserted porous plug as herein described, the plug will act as a reservoir for the alkali metal. The force of adsorption plus the capillarity of the pores, will be greater than the force of gravity thereby preventing the alkali metal from dripping. The lamp could operate in any position as well as in gravity-free environment. Since the alkali metal will return to the reservoir of the plug, the electrical lamp discharge will not terminate on the jacket thus avoiding shorting by an alkali metal film shorting across the lamp causing the lamp fail. Startup will be more uniform and faster since there will be no alkali film on the jacket.

We wish it to be understood that we do not desire to be limited to the exact details of construction shown and described, for obvious modifications will occur to a person skilled in the art.

What is claimed is:

1. An alkali metal vapor lamp comprising a tubular elongated envelope, said envelope having a pair of electrodes sealed into opposite ends of said envelope, one end of said envelope having a metallic exhaust tubulation sealed thereto, said one end being cooler than said other end during lamp operation, an ionizable medium including metal vapors sealed within said envelope, said metal vapors being selected from the alkali metals Sodium, Potassium, Rubidium and Cesium, and a means for preventing alkali metal deposits in the interior of said envelope comprising a porous Beryllium plug having a surface coating of ceramic oxide positioned with and adjacent to the cooler end of the envelope.

2. A device of the kind set forth in claim I wherein said ceramic oxide is beryllium oxide. 

2. A device of the kind set forth in claim 1 wherein said ceramic oxide is beryllium oxide. 